Paradoxically, protein biosynthesis proceeds with a fidelity greater than the fidelity of base-pairing. The paradox is resolved by experiments which demonstrate the existence of a proofreading reaction in aa-tRNA binding to poly U-programmed ribosomes. Further experiments are proposed to demonstrate the generality of the proofreading reaction, to define the role of this reaction in maintaining the high fidelity of protein biosynthesis, and to investigate the mechanism of proofreading. The general occurence and specificity of proofreading will be investigated with poly A and poly C-programmed ribosomes. The role of proofreading will be studied by attempting to co-relate the effects of abnormal ionic conditions, antibotics, and ribosomal mutations on the proofreading reaction, with their effect on the fidelity of protein synthesis. The mechanism of proofreading will be examined by determining whether the rejection of errors occurs prior, or subsequent to the movement of aa-tRNA to the ribosomal A-site. Other experiments are planned to investigate the chemistry of the aa-tRNA binding protein, EFTu. The topography of its guanine nucleotide binding site will be investigated by photoaffinity labeling. The control of its GTPase activity will be investigated by controlled denaturation of the protein.